With respect to carbon dioxide emission, the proportion of civil engineering and construction in overall industry is very large, and it is desired to reduce its environmental load. On the other hand, carbon dioxide discharged from cement industry derives mostly from fuel during calcination or from decarboxylation of limestone as the starting material. Accordingly, in order to reduce the carbon dioxide emission, it is the most effective method to reduce the calcination amount of cement clinker, and it is extremely important to promote utilization of various mixed cements.
Further, with a view to constructing durable concrete structures, it is desired to prevent bleeding resulting from segregation and to suppress heat generation by hydration. Particularly, in recent years, the performance required for concrete is diversified, and for the purpose of rationalization of application, various types of high fluidity concrete have been proposed which require no compaction and which have self-filling properties (“Report I of JCI Ultra Fluidity Concrete Research Committee” (May 1003) and “Report II of the same” (May 1994), published by Japan Concrete Institute).
Such high fluidity concrete tends to undergo segregation if the unit quantity of a binder such as cement is less than about 500 kg/m3, and accordingly, it usually has a unit binder quantity of at least about 500 kg/m3. And, high fluidity concrete having a large unit binder quantity has a problem that the hydration heat value is large and the environmental load is also large.
In order to solve such a problem, high fluidity concrete has been proposed in which fine powder of limestone is used in substitution for a part of the binder (JP-A-5-319889). The fine powder of limestone shows no substantial hydraulicity, and accordingly, the high fluidity concrete in which fine powder of limestone is used in substitution for a part of the binder, has a merit such that it imparts segregation resistance without bringing about unnecessary heat of hydration. However, limestone is a valuable natural resource for our country having small resources, and its utilization merely for admixing it to concrete, is likely to lead to depletion of the resource. Accordingly, there are presently many voices for more effective utilization of limestone.
Further, limestone-admixed cement has a problem that it is likely to be neutralized, and it is poor in providing initial strength. Neutralization is important since it relates to durability of a ferroconcrete structure, and the initial strength is important, since it deeply relates to unmold cycle and thus is important for shortening the application period. Further, the neutralization is an important deterioration factor influential over the durability of ferroconcrete, and in neutralized concrete, the reinforcing steel will be corroded, and there will be a danger of falling of concrete fragments. At present, it is desired to develop a cement composition which is excellent in providing durability and initial strength and which is capable of providing strength equal to one where usual Portland cement is used alone, even if the admixture is used in combination in an amount exceeding 30%.
On the other hand, blast furnace slag discharged as industrial waste from steel plants is widely used in the cement concrete field. The blast furnace slag is generally classified into quenched and vitrified so-called granulated blast furnace slag and air-cooled and crystallized so-called slowly cooled blast furnace slag. Among them, the granulated blast furnace slag has alkali latent hydraulicity, and one pulverized to the same level as cement, or more finely than cement, is used as a starting material for blast furnace cement.
Vitrified granulated blast furnace slag has excellent latent hydraulicity whereby even when admixed in a large amount to cement clinker, its long term strength will not decrease, and its studies are being made in various fields of e.g. high strength concrete and high fluidity concrete (“Applicability of Fine Powder of Blast Furnace Slag to High Strength Concrete”, Kenichi Yasudo et al., papers reported at the 45th Cement Technology Convention, pp. 184-189, 1991, etc.).
The vitrified granulated blast furnace slag has excellent latent hydraulicity whereby the strength will not decrease for a long period of time even when it is admixed in a large amount to cement clinker. However, on the other hand of exhibiting such high strength, the vitrified granulated blast furnace slag has had a problem such that heat generation by hydration and autogenous shrinkage tend to be substantial. Such heat generation and autogenous shrinkage are factors which induce cracking, and they are phenomena not desirable when durable ferroconcrete structures are to be constructed.
On the other hand, slowly cooled blast furnace slag is called also by another name i.e. crystallized slag or ballas and is one showing no hydraulicity. Accordingly, it was used mainly as a roadbed material, but recently, reclaimed aggregate has become to be preferentially used as a roadbed material. Thus, it is likely to lose the conventional application, and its useful application is still being sought (“Application of Blast Furnace Slag to Cement Concrete”, Akihiko Yoda, Inorganic material, Vol. 6, pp. 62-67, 1999), “Law Relating to Promotion of Utilizing Reclaimed Resource, so-called recycle law”, October 1991).
In recent years, a problem relating to durability of concrete has been taken up as a serious problem, and guidelines, principles for standardization, etc., relating to durable concrete have been published by various academic societies. Particularly, the water/cement ratio of concrete is substantially influential over the durability, and accordingly, for the purpose of reducing the unit quantity of water, frequency of use of a high performance water reducing agent or a high performance AE water reducing agent has rapidly increased, and guidelines therefor have also been issued.
However, there is a problem such that as the unit quantity of water is reduced, the change with time of consistency tends to be large, and the fluidity tends to decrease, and at present, there is no fundamental solution to this problem. It is particularly in the case of high fluidity concrete that the problem of the fluidity decrease is taken up as a serious problem.
High fluidity concrete is a concrete developed not to be susceptible to an influence due to the quality of workmanship, and its self-filling property is the most important characteristic. And it is required to maintain the fluidity during the transportation from the ready-mixed concrete plant to the application site and further for a certain period at the application site. However, due to some troubles at the application site or a traffic jam during the transportation, it may frequently take time over the prescribed time, whereby the fluidity of concrete tends to be outside the prescribed standard.
In such a case, there is no other means than carrying out so-called re-fluidizing treatment i.e. by additionally adding a high performance AE water reducing agent or the like to have it re-fluidized. However, at present, such re-fluidizing treatment can be done only by a skilled hand. Accordingly, it is strongly desired to develop concrete excellent in the fluidity-maintaining performance.
On the other hand, from the viewpoint of an environment problem, a large expectation is present with respect to reduction of hexavalent chromium which presents adverse effects against human bodies. A method has been proposed to reduce the amount of elution of hexavalent chromium by a reducing agent or adsorbent thereof. However, such a material is too expensive to be used in the field of cement concrete and is not substantially practically used.
Under these circumstances, the present inventors have conducted various studies on effective utilization of slowly cooled blast furnace slag and as a result, have found that a slowly cooled blast furnace slag powder has a function of suppressing neutralization, is excellent in the performance in maintaining fluidity and segregation resistance, has small-autogenous shrinkage and can be made to be high fluidity concrete having low heat generation by hydration, and shows an effect to reduce hexavalent chromium under a prescribed condition, and further, by using a cement composition comprising a Portland cement having a C3S content of at least 60% and a slowly cooled blast furnace slag powder, it is possible to make a cement composition excellent in the initial strength-providing property and having little environmental load, whereby the conventional problems can be solved, and the present invention has been accomplished.